Heating with Compost

Heating a building with compost sounds like a new, innovative idea – but it’s nothing new. In the past, farmers have employed this heating method by allowing the bedding and manure from livestock to compost in the barn over the winter to help keep their animals warmer. The technique only added a few degrees to the barn’s indoor temperature – but that makes a big difference on a cold winter night.

As the cultural shift toward sustainable living has grown, composting has experienced a surge in popularity. As a result, the practical advantages of composting are being re-addressed, particularly in regard to energy savings – hence the interest in heating with compost. In this post, we discuss compost heating methods and their practicality. We’ll also ask you what you think.

The concept. I recently saw a concept for a yurt with compost bins around the perimeter. The idea was that the compost would help heat the building – but I never saw how well it worked, and it looked a bit cumbersome. Then again, if you live in a yurt, “cumbersome” probably has a different meaning to you. Perhaps it’s more practical than I’m imagining.

This got me thinking about heating my wife and I’s, future home and some of the issues involved in a compost heating system. Once I can experiment, I’ll post more in-depth information, but to whet your appetite, here’s what I’m thinking so far: aerobic compost produces tons of heat, water vapor, and carbon dioxide. And when I say heat I mean it – even a small pile can reach 140F. Did I mention that compost can spontaneously combust? Yeah, we’ll have to deal with that too. It should be possible to use the heat generated by the compost to heat a home safely and effectively. Even if the compost only offsets some of the fossil fuel heating costs, it could be a huge win.

Venting the CO2. I grew up in Vermont, where winters are dry and dusty, leading to a host of health issues. While North Carolina winters are much more temperate, they’re still a bit dry. For a successful compost heating project, it’d be nice to use some of the water vapor to humidify the dry winter air. Still, that would have to be a secondary goal, because it seems that the biggest challenge to compost heating is venting the CO2 and smelly gases without losing all of the heat that’s generated. The CO2 is dangerous, and the smell from trace gases is unpleasant. Somewhere along the line, the gas must pass through a biofilter (fancy talk for “finished compost”). I’d like the design to be oil/electric-power-free, but even if some is needed it’s still more energy efficient to run a blower than a heater. I’m guessing the best design is a forced aeration in vessel system (put a bunch of composting material in a barrel and blow air through it).

Coil-based heating. One approach would be to heat water by placing a coil at the core of composting material and circulating water through. As I found out in cooking with compost, opening up the pile causes a huge drop in temperature and it’s slow to reheat. In addition, I’d fear wicking away too much heat would drop the core temperature and slow the thermophilic bacteria (compounding the heat generation issue). This approach may still be worth investigating, though.

Using a heat exchanger. The heat is best captured by venting the hot compost gases through an air-to-air heat exchanger. The moisture may be a problem, so perhaps it should first run through a dehumidifier. Keeping a well-sealed air envelope inside the house and providing additional heat from compost is feasible; whether it’s economically viable remains to be seen. If you remove cheap oil/coal-based energy from the equation, a lot of things become economically viable.

Feedback needed. Has anyone tried heating with compost? Thoughts? At least I’m not the only one.

UPDATE Feb2011: This has been a much more popular post than I expected. I have an experiment designed but am traveling (work, no fun I promise) and will have some results in the coming months along with a prototype design.

Good day. I hope you are well. I have just read your article on compost heating. i have for a while being gathering info on producing my own mushrooms, but really want to stay off grid. i live in Memel,South Africa, and our winters go below 0 and sometimes snow. Mushrooms cant go below 10 degree celsius. i am considering buiding with straw for its insulative properties. So my floor would need heating in winter and compost sounds great. Do you have any info on doing such a floor instalation? Keep well. Timothy.

That’s awesome Timothy. I wish you the best of luck. I have seen a few designs online-most involve making a large compost pile outside and running metal tubing through it. Circulating water through the tubing transfers some of the heat and the warm water is used to heat the flooring. You will have to run many batches of compost through the winter since the hot phase of composing only lasts a few days/weeks. There are two heat exchange points (water warming in the compost and water warming the floor) so you have to ensure its built so-as to transfer as much energy as possible at each of those points.

Is it sunny in the winter? Passive solar can be very effective at heating in the winter. Finally consider making your walls as thick and dense as possible. That will give you thermal mass to help minimize the temperature change inside (it takes a long time to heat up or cool down).

I have been doing some research on utilizing an insulated chimney to create adequate draw of fresh air through a large compost pile. A well insulated chimney on top of the pile allows me to cover 70% to 80% of the upper portion of the pile, leaving only the widest bottom portion uncovered to allow for air to travel into the pile. The hot air going up the chimney draws fresh air into the pile from the uncovered base. Covering the upper 3/4ths of the pile traps a significant portion of the moisture in the pile, helping to prevent it from drying out too quickly.

Many of the principles are the same for when burning wood. A chimney is necessary to create draft to provide a steady supply of oxygen to the burning fuel. An insulated chimney significantly increases the amount of draft by creating a greater differential in temperatures between the air outside the chimney, and that within. In other words, the hotter the air, and the longer it stays hot, the more draft that will be created. In the long term it is far less expensive to build a chimney than to power a fan. Also, solar heating can be incorporated to further increase the amount of draft created during certain hours.

From an article on chimneys: “The chimney works with the stove or fireplace in a kind of feedback loop. Heat in chimney makes draft, which pulls in more combustion air, which makes the fire burn hotter, which delivers more heat to the chimney which makes more draft and so on. An insulated chimney makes more draft with less heat.”

This system is a very old one. Back in the day this was commonly used on farms. They would dig a pit 8ft. X 8ft x 8ft. Using tubing, coil as much as possible then fill hole with wood shavings. Pump the water with a circulation pump and within 10 days produce heated water over 110 degrees. Some farmers would also place a barrel in the middle filled with manure and produce methane gas. Methane gas is able to be compressed and they would have farm equipment converted to run on this gas.

good stuff – thanks for the info. There’s a farm near my folks house that runs their entire operation on methane from digestion of cow manure. I’ve read about farms also allowing bedding to compost in the barn to keep it a bit warmer over the winter.

I wonder about using this kind of heat to heat a hot house during winter cold, perhaps with some further heating from the ears of rabbits/concentrating solar mirrored heating?/solar powered fans to push the air around with back up car batteries?

Maybe our old grass clippings could give us some winter heat. Add rabbit dung carefully to produce fertilizer for the hot house later in the spring.

Those are all great ideas Ron. Just raising the temperature and humidity a little in a greenhouse makes a huge difference. I used to keep a 55gal drum of hot compost in ours in the winter – the biggest challenge was getting a new batch to heat up since a 55gal drum doesn’t produce a lot of excess heat. Rabbits would help too. Sadly I don’t get any sun in the winter but if you do then passive solar is an awesome way to go. Even just a huge drum of water will absorb heat all day and radiate it out all night. Solar concentrators and heat exchangers can help you focus that heat some place that you can store it (like a huge tank of water).

My father told me that farmers used to pile manure/bedding up against the outside of the north side of the barn to block the coldest winds and provide heat through the walls of the barn. One of Michael Pollan’s books talks about leaving manure/bedding in the barn over the winter to compost and heat the barn. I’ve even read of farmers getting crops started early by burying hot compost and planting on top of it. The heat warmed the soil enough to germinate seeds.

I don’t have a patent on it, I know that. I can’t imagine it would be patented and certainly shouldn’t be allowed to be patented. There is a product or two out there that uses compost for heating but I’d love to see more. Are you doing a write up on your science expo project? If so I’d love to see it if you don’t mind sharing a link here.

Goodday, I’m thinking of a simple method to heat a small space for winter, 6ft by 6ft. I thought a compost barrel on its side with the barrel in the house and the opening outdoors would be a simple method, the heat would just conduct into the house but hopefully only vent outside. And it seems if I made it a roller barrel I could get a lot more heat from it. If I make it a roller then I would have more trouble insulating well between the barrel and the wall. Have you had any thoughts on such a system or seen it anywhere in use?

The heaters I’ve seen in practice were huge – at least 5-10 yards of material. What I found was that a 55gal drum wasn’t big enough to generate usable heat. All of the heat it produced was needed to keep the compost warm and keep the reaction going. I figured out the amount of energy one could siphon off once and concluded that you need at least a yard of material to bother with it. Finally my 55gal drum digester would need to be changed over every 5 days or so because all of the consumable material was composted (it still needs to rest before being good garden compost but cool off rapidly). I would reach internal temperatures of 170F but only sustain that for 1-3 days.